SUMMARY: The effect of dietary chlortetracycline on the serological type and resistance of the streptococci found in the caeca of chickens has been investigated. In the control birds where only chlortetracycline-sensitive streptococci were present initially. predominated and (mainly the proteolytic variant) was present in small numbers representing three serological types (types H69D5, D15 and D76). Administration of chlortetracycline, whether at a low concentration throughout life or intermittently at a high concentration, led to the emergence of a highly-resistant non-proteolytic strain of (type H69D5) which became predominant in the antibiotic treated birds. Withdrawal of chlortetracycline was followed by disappearance of type H69D5 and the reappearance of types D15 and D76 which were proteolytic. Type D15 remained sensitive to chlor-tetracycline, but type D76 had become more resistant and eventually became the predominant streptococcus in all the chickens under observation. The serological types of identified in these chickens are also commonly found in the human intestine.


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